(1) Field of the Invention
The present invention relates to a communication apparatus, and more particularly, to a communication apparatus interposed between an exchange and a transmitter or between transmitters in a digital transmission system such as an optical communication system.
(2) Description of the Related Art
In optical communication systems, communication apparatuses including multiplexers are connected between an analog or digital exchange and an optical communication network. The communication apparatuses can be categorized according to frequencies to deal with, i.e., according to predetermined transmission speeds. Generally, each communication apparatus is mounted to a casing or a shelf, and the casing or shelf is mounted on a frame or a rack.
The communication apparatus has a plurality of electronic circuit boards detachably mounted thereto. Specifically, the casing for the communication apparatus has a plurality of shelves therein, and guide members are formed at each of upper and lower walls of the shelf and extend in a direction along which the electronic circuit board is slid for attachment or detachment. The electronic circuit board is inserted along the guide members, whereby the circuit board is mounted to the shelf of the casing. At this time, electrical connection is established between the electronic circuit board and a back wiring board arranged at the rear of the shelf. Namely, mating connectors are formed respectively at each electronic circuit board and the back wiring board, and mechanical connection of these connectors achieves electrical connection among the electronic circuit boards via the back wiring board.
Further, it is necessary that signals be transferred between an electronic circuit board and another electronic circuit board in the same electronic device or an external electronic device. To this end, connectors connected to office cables are fixed to the frame, a small-sized casing with a back wiring board, which is called a block, is fixed at the position where the office cable connectors are fixed, and an electronic circuit board is mounted to the block, whereby electrical connection is achieved between external connectors formed on the electronic circuit board and the office cable connectors fixed to the frame.
Some communication apparatuses include duplex electronic circuit boards, for example, two power supply packages, for redundant operation or to improve the reliability. When one of the power supply packages fails, it is replaced with a new one. On the other hand, when the number of channels is to be increased, for example, an additional electronic circuit board(s) need be connected. Thus, in cases where electronic circuit boards are replaced or newly added, it is necessary that the electronic circuit boards be detached from or attached to the connectors associated therewith.
When an electronic circuit board is detached or attached, the power supply voltage is liable to temporarily lower. For example, where two power supply packages, one for current use and the other for standby reserve, are used for parallel operation, each package bears half of the load current. If the power supply package for current use, for example, is detached, the power supply package for standby reserve is required to bear a load current which is twice the current it has been bearing. In such a case, although a DC/DC converter of the power supply package for standby reserve performs a control action such that a twofold load current flows, the control action is subject to time lag, possibly causing an instantaneous voltage drop. As a result, load-side packages may fail to function properly, due to the instantaneous voltage drop, thus causing a channel error and lowering the quality of channels. Further, when a package is additionally connected to the load side, a very large rush current flows through the package for a short time, similarly causing an instantaneous drop of the power supply voltage and a channel error. To eliminate the drawback, conventionally large-capacity bypass capacitors are directly soldered to sheet connectors associated with the respective back wiring boards of the power supply packages for current use and standby reserve, to absorb sudden fluctuations in the power supply voltage.
The communication apparatuses are classified according to frequencies to deal with, as mentioned above; for example, they include apparatuses associated with a first-order speed of up to 2 Mb/s (megabits per second), apparatuses associated with a second-order speed of up to 8 Mb/s, apparatuses associated with a third-order speed of up to 34 Mb/s, apparatuses associated with a fourth-order speed of up to 140 Mb/s, and apparatuses associated with a speed higher than the fourth-order speed. These apparatuses can be directly connected to one another; in general, electrical signals are transferred among the apparatuses via digital distribution frame devices (hereinafter referred to as "DDF devices").
The DDF device is used for the following reasons: In cases where apparatuses from different manufacturers are used, connectors for wiring, terminals, and types of cables used in the apparatuses often differ from manufacturer to manufacturer, requiring interfacing points for resolving the differences. Also, when apparatuses are installed at different times, interfacing points that permit the wiring to be switched for the individual apparatuses are required in order to enhance the efficiency of the installation work. Further, since rewiring after installation consumes a great deal of man-hours and cost, interfacing points that facilitate rewiring, such as jumper lines, are needed. In many cases, connections of signal lines are performed at the back of the apparatuses; therefore, signal monitoring during on-site adjustment or maintenance need be conducted at the front side, not the rear side, of the apparatuses. To meet the requirements, the DDF device provides a line switching function by means of jumper lines and a signal monitoring function, and thus is indispensable to the communication apparatuses.
DDF devices can be generally classified into three types, i.e., low-frequency 75-.OMEGA. DDF device, low-frequency 120-.OMEGA. DDF device, and high-frequency DDF device. The low-frequency 75-.OMEGA. DDF device and the high-frequency DDF device are used for the connection with unbalanced coaxial lines, while the low-frequency 120-.OMEGA. DDF device is used for the connection with balanced paired wires. Thus, even in the case of low-frequency devices, customer's interface may be of 75-.OMEGA. impedance or 120-.OMEGA. impedance.
In the low-frequency 75-.OMEGA. DDF device, input/output signal lines and jumper lines are both coaxial lines and thus coaxial connectors therefor occupy a large area, making it difficult to reduce the size of the device. Apparatuses for dealing with low-frequency input/output signals require two types of connectors, i.e., flat connectors to be connected to paired wires and coaxial connectors to be connected to coaxial lines, thus requiring a large area for the connectors and increasing the size of the apparatuses. In conventional DDF devices, therefore, connectors connected to the apparatuses are limited to wrap-type flat connectors, for example. In the case of the low-frequency 120-.OMEGA. DDF device, paired wires therefrom are directly wrap-connected to connectors of the apparatuses. For the low-frequency 75-.OMEGA. DDF device, coaxial lines therefrom are each extended to the vicinity of the corresponding apparatus and connected to a coaxial-line/single-conductor converting member, the two lines from which are wrap-connected to connectors of the apparatus. Alternatively, the apparatus is designed to have a 75-.OMEGA. impedance/120-.OMEGA. impedance conversion function, and switching is effected according to the impedance of wiring.
In the conventional communication apparatus, however, the connectors connected to other electronic circuit boards in the apparatus and the connectors connected to external devices are fixed to the frame, making it difficult to perform rewiring after the wiring inside the frame is once completed, and rewiring work, if performed, is very costly. Further, with regard to the connection with other electronic circuit boards via the back wiring board, once the wiring is carried out, the position of the electronic circuit board is uniquely determined, because the connectors associated with the shelf are determined or main signals pass the back wiring board. Furthermore, since the back wiring board is specially designed, the cost is high, and it is difficult to change the shelves.
The bypass capacitors, which serve to suppress channel error caused by instantaneous fluctuation in the power supply voltage at the time of attachment or detachment of a package, are directly soldered to the sheet connectors on the back wiring board. It is, therefore, difficult to replace the bypass capacitor when the service life thereof expires or the bypass capacitor fails, which may lead to long-term disconnection of channels.
In DDF devices for interconnecting apparatuses, particularly the 75-.OMEGA. DDF device, coaxial lines are extended to the vicinities of the apparatuses, requiring much space for the cables. Further, since the number of coaxial lines is large and a coaxial-line/single-conductor converting member must be provided between each coaxial line and the apparatus, the efficiency of wiring work is low. If such connection work is performed by an inexperienced person, error may occur due to loose contact. Apparatuses for dealing with low-frequency input/output signals have a 75-.OMEGA. interface or 120-.OMEGA. interface; therefore, they need to have the impedance conversion function. Further, since wires connected to the low-frequency 75-.OMEGA. DDF device are all coaxial lines, they occupy a large area, and thus the number of channels is small due to limited mounting space.